More and more couples are waiting longer to have children. Do you see this as a problem? Do you see humans adapting to have births later, especially as
our life span increases?
Women are born with a certain number of starter cells for eggs, which gradually decline. Evolution has acted to stop production of eggs at menopause: it's
not accidental. Lots of evidence points to the optimal time for a woman having a baby to be between 20-25. Now many women delay that because of careers.
We do know that it increases the risk of miscarriage and birth defects. But whether, over time, we will adapt to this, I don't know. We don't know enough
And doesn't the age of male sperm matter as well?
It's typical -- people study a lot of things in women and quite often haven't gotten around to studying it in men. It's long been known that the probability
of having a baby with Down Syndrome increases with a woman's age. But a man's age contributes, too. And sperm counts decline as men get older.
But one thing that hasn't been studied is the fact that the increasing risk of reproduction with age may be related to copulation. As people get older, it
happens less and less frequently. So the probability of having an aging sperm fertilize an egg, or an aging egg being fertilized, increases. This could be
the connection between age and Down Syndrome -- the decline in frequency of copulation.
What do you think of assisted reproduction, like IVF?
When you deposit semen directly into the womb, it bypasses the filter of the neck of the womb, which produces mucus, and the sperm have to swim up.
There's a selective process -- we know that sperm are filtered, and only healthy sperm get through that barrier. If you inject sperm directly, you bypass the
filter. There's also intracytoplasmic sperm injection, injecting sperm directly into an egg. There's no selection, it's just a sperm picked at random. You
haven't had the filtering effect of the neck of the womb, or any other filter that happens before the sperm gets to the egg. It comes back to the question
of why we need so many sperm: there's a filtering that helps the sperm reach the egg in top condition.
In your book, you discuss the rise of C-sections. Why should we be worried?
It's really quite frightening. There's an epidemic: around one in three women in the US now gives birth by C-section. Twenty years ago, the World Health
Organization said C-sections shouldn't be above 15 percent, which I thought was a high figure anyway. They gave that as a target, and we're more than
double that. The highest figures are in Asia--in China, it's around 45 percent and growing. But apparently, there are propitious birthdays--quite often the
C-sections are fixed so the baby is born on that specific day.
There's a huge downside to this. One effect of C-sections is removing the selective effect. If we get into a situation where 50 percent of women are having
C-sections, are we going to remove the selection on overly large heads? What we're doing is stopping natural selection. And generally, cesareans take place
too early. The baby is born prematurely, and premature babies have all kinds of problems. And with every operation, there is a risk. Women should be
advised that this is a pretty dangerous option.